I have no problems with this being postponed to later release.
However I would clarify some point from your reply.
By "session" I was just meaning a Denemo session ie open Denemo, do some work, save, close (or crash). So I suspect that a similar situation could occur on Linux too. If you use denemo 10 times before you reboot you would have 10 sets of temporary files. Ok when you do reboot they go on Linux but they stay on Windows until you make an effort to get rid of them.
The issue you mentioned about the old pdf files. I think that problem came in about 0.9.0. Before that it was OK. I have got used to checking the dates on the files!
As for volunteering, If I ever get back into programming I might might. But don't hold your breath, I've been out of main stream computing for 8 years and no longer even have a development environment at home. Meanwhile, I don't mind doing the odd bit of testing every now and again. I use denemo quite a bit.

Yes, those of us in the know resort to this. The less techie way is to do File->Export->LilyPond which gets the file into a known place. This would not debug problems with things like Print Part (or indeed with printing a customized layout), for those selecting copying and pasting from the LilyPond window will work I think.
Problem a) is Windows specific, it is deleted on GNU/Linux on next boot (I guess you mean this by session?), if it is a problem we could put in something on Denemo startup to delete old Denemo* files I guess.
There is some real issue about seeing old pdf output when the new pdf generation has failed. Oh, and a bigger issue on Windows is that the error messages from LilyPond are not detected so the cursor is never moved to the error point, and it does not even tell you that it can't do this on Windows :(

CheckScore is just a first stab at a script to check some of the things that have bugged me from time to time, it could do with a volunteer :)

This one is more of an issue rather than a bug.
The temporary folder on Windows has moved to <user>\AppData\Local\Temp\DenemoXXXXXXXX where XXXXXXX appears to be some random number (some positive, others negative and between 8 & 10 digits seen so far).
While this is probably a more correct place to put temporary data than the old .denemo folder, it is much harder to find. That shouldn't be an issue if the data truly was temporary but ...

a) Data is not deleted at end of session so it builds up.

b) When things go wrong, I find that the best way to find the problem is to go to the temporary folder and double click on the denemoprint.ly file to pass it through lilypond directly. That generates a log file that usually shows where problem is. Then I usually have to look at .ly file with something that has line numbers before I can work out where the fault is in denemo. At the moment denemo is still a bit deficient in the area of finding faults but CheckScore looks like a step in the right direction. Perhaps when CheckScore gives more information about what the problem is rather than just where it is, then the .ly file can be treated as a truly temporary file.